1. Field of the Invention
This invention relates to a process for treating aqueous solutions containing phenols.
2. Description of the Prior Art
In Japan, some of the phenols are designated as special chemical substances by the Law of Labor Safety and Hygiene, and the content of phenols in waste waters is strictly regulated from the viewpoint of environmental protection. Accordingly, a variety of processes have been devised for removing phenols. However, all these processes have some faults.
As one of these processes, the activated sludge process can be referred to. This process is employed as a process for removing phenols from aqueous solutions containing relatively low concentration of phenols. However, it has a problem that it cannot be employed when the concentration of phenols is high. Also, the post-treatment of sludge is troublesome. A further fault of this process consists in that the phenols cannot be reused as a resource.
As another process, there has been proposed a process of extracting phenols from phenol-containing aqueous solutions by using, as an extracting solvent, an organic solvent such as hydrocarbons (for example, benzene and the like), ethers (for example, isopropyl ether) or alcohols (for example, octanol). However, if a hydrocarbon is used as the extracting solvent, the efficiency of extraction is quite low. If an alcohol such as octanol or an alcohol diluted with hydrocarbon is used as extracting solvent, a considerably long period of time is necessary for separating the extract solution from the water treated, even though their ability of extraction is fairly high. Further, this process has a problem that, when the extracted phenols are released from the extract solution by the use of an alkali, a loss of the alkali occurs due to its transfer into extracting solvent and the extractant must be washed with water which complicates the procedure. Having these many faults, the process of using hydrocarbon or alcohol as extracting solvent is difficult to employ industrially.
When an extraction process is to be adopted, therefore, it is usual to use an ether such as isopropyl ether as the extracting solvent, as is mentioned in British Pat. No. 876,452. However, ethers are quite difficult to handle in that they form an explosive peroxide upon contact with air and have a low flash point. Further, when the phenols extracted into the ether, used as extracting solvent, are released with an alkali, separation of the liquid phases is quite slow. Further, when the phenols extracted into the extracting solvent are recovered by distillation at elevated temperature, ethers have a lower boiling point than phenols, so that it is necessary to distil out the ether first and thereafter to recover the phenols again by distillation. Such a process cannot be said to be advantageous industrially because it requires heat energy for distilling the ether.
In U.S. Pat. No. 2,812,305 there is proposed a process for removing phenols by using a mixture of aromatic hydrocarbon and 5-ethyl-2-methylpyridine as an extracting solvent. However, this process has low efficiency of extraction. In addition, this process has various faults. For example, 5-ethyl-2-methylpyridine has a boiling point of 177.8.degree. C./747 mm Hg which is very close to the boiling point of phenol (181.75.degree. C./760 mm Hg), so that it is quite difficult industrially to separate phenol from the extracting solvent by the distillation of the phenol-containg extract solution. Even if it is possible to separate them by distillation, phenol must be recovered by distillation after 5-ethyl-2-methylpyridine has distilled out because 5-ethyl-2-methylpyridine has a lower boiling point than phenol, and a large quantity of steam is necessary for distilling 5-ethyl-2-methylpyridine.